Gas turbine engines, such as those used to power modern aircraft, include a compressor for pressurizing a supply of air, a combustor for burning fuel in the presence of high pressurize, compressed air to generate and accelerate high temperature, high velocity combustion gases, and a turbine for extracting energy from the resultant combustion gases. The combustion gases leaving the turbine are exhausted through a nozzle to produce thrust to power the aircraft. In passing through the turbine, the combustion gases turn the turbine, which turns a shaft in common with the compressor to drive the compressor.
As the hot combustion gases pass through the turbine, various turbine elements, such as the turbine stator vanes and turbine rotor blades of the turbine, are exposed to hot combustion gases. In order to protect these turbine elements from exposure to the hot combustion gases, it is known to cool the turbine blades and vanes. In order to facilitate cooling of the blades and vanes, it is known to form the turbine blades and vanes with complex systems of internal cooling passages into which compressor bleed air, or another cooling fluid, is directed to cool the blade or vane. The cooling air exits the blade/vane through a system of holes arranged in such a manner that the exterior surface of the blade/vane is cooled, and is then passed out of the engine with the rest of the exhausted combustion gases.
In some turbine blade/vane embodiments, the cooling air exit holes are arranged in a specific pattern on various facets of the blade/vane airfoil to create a surface cooling film. The surface cooling film creates a layer of cool air, which insulates the airfoil from the hot combustion gases passing through the turbine. In order to ensure that the surface cooling film properly forms, various shaped exit holes are precisely located and bored at various angles on the surface of the airfoil. Thus, after manufacture it is necessary to inspect the blades and vanes to ensure the holes are properly positioned.
Conventional inspection systems include a fixture for holding the turbine blade/vane being inspected, a video camera, and a computer for controlling the inspection process and processing the video camera images. Generally, conventional inspection systems require inspection of each cooling hole from a gun-barrel view, which typically also requires the use of a five-axis coordinate measuring machine (CMM) for orientating the element and stepping the video probe from hole to hole. The turbine vanes and blades may, for example, have as many as 200 to over 300 cooling holes, each of which must be individually inspected.
Conventional inspection systems implement a step and stop process inspection, wherein the video camera is moved from hole location to hole location and positioned in a stationary relationship relative to the hole for a period of about 1.5 to 2.0 seconds before moving on to the next hole. This dwell time is needed for the video camera and the target hole to synchronize position, for the video camera to image the target hole, and the computer to analyze the dimensional measurements and output results. The video camera has a low frame rate capability, typically only 30 frames per second. Typically, inspection of a single airfoil may take as long as ten minutes, depending upon the number of holes and also the time required in initial part probing. Part probing is required to properly position the part to be inspected in the workpiece fixture prior to initiating the actual hole inspection, which in conventional practice can take from about 1.5 minutes to over 3 minutes.
Conventional calibration techniques for inspections systems include using two separate artifacts to measure all of the calibration parameters. In addition, those calibration artifacts require a dedicated, fixed location of the machine table and a hardwired connection to a power source. Use of a dedicated location to the side of the main work area requires extra reach for at least one of the machine axes. As a result, motors, actuators, bearings and scales are longer, the machine is heavier, and more space is needed solely to accommodate for calibration. Along with the added installation expense, there are reliability costs and safety concerns associated with the wires routed through the articulating joints of the machine, electrical contacts and grounding. Therefore, there is a need for a simplified and more reliable calibration system. This disclosure is directed to solving this need and provides a way to reduce the cost and complexity of a calibration device for inspection systems.